Reduction of noise



Dec. 27, 1938. R M SMITH 2,141,229

REDUCTION OF NOISE Filed Sept. 50, 1956 2 Sheets-Sheet 1 2 Sheeis-Sheet2 Filed Sept. 30, 1936 Patented Dec. 27, 1938 REDUCTION OF NOISE RogersM. Smith, Merchantville, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application September 30, 1936,Serial No. 103,398

6 Claims.

This invention relates to the reduction of the noise produced in radioreceivers and similar apparatus by impulses extraneous to the desiredsignal impulses, and has for its principal object the provision of animproved apparatus and method of operation whereby such noise issubstantially eliminated or greatly minimized.

In the development of radio receivers and the like, many sources ofelectrical impulses have come to be recognized as productive ofundesired noise. Among these may be mentioned static, electric streetcar lines, internal combustion engines, electric elevators and otherdevices which involve the control and utilization of electricity. Theelimination of the undesired effects of these extraneous impulses is aproblem susceptible of a number of different solutions many of whichinvolve the use of complicated and expensive apparatus or are notaltogether effective.

In its broader aspects, the solution herein disclosed involves theprovision of (1) means for maintaining a substantially constant signallevel, (2) means for limiting the output capabilities of the apparatus,and (3) means for adjusting the output limiting means to permitoperation at different signal levels. Stated more specifically, theapparatus of the present invention may include a reflex circuit in whichan audio frequency amplifier is associated with means for automaticallycontrolling its volume, a control member adapted simultaneously toadjust the signal input of this audio frequency amplifier and the screengrid potential of the output stage of the amplifier and a tonecompensation circuit associated with one or more stages of theamplifiers. As will hereinafter appear, certain rearrangements of thesevarious elements may be made without defeating the objects of theinvention.

Among the further objects of the invention are (1) the provision of animproved automatic volume control system which operates with asubstantially fiat characteristic, (2) the provision of an improvedcontrol means for correlating the signal level and output capabilitiesof a signal amplifying channel and (3) the provision of an improvedreflex circuit which is not subject to some of the difiicultiesheretofore encountered in the use of such circuits.

The invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawings and itsscope is set forth in the appended claims.

Referring to the drawings Fig, 1 is a wiring diagram of a radio receiverprovided with a reflex circuit which includes an automatic volumecontrolled audio frequency amplifier having its signal input levelcontrolled in accordance with the screen grid potential of the amplifieroutput stage, and

Fig. 2 is a similar diagram of a modified radio receiver wherein thevolume levels at the reflex circuit and at a following audio frequencystage are simultaneously adjusted. It will also be noted that somewhatdifferent tone compensation systems are utilized in the apparatus ofFigs. 1 and 2.

The circuit of Fig. 1 includes a signal pick-up device such as anantenna I0 from which signal impulses are supplied to a utilizationdevice such as a loudspeaker H. Between the antenna If! and loudspeakerII is interposed a signal amplifying channel which includes (1) radiofrequency stages I2 and i3, (2) an electron discharge device M (RCARadiotron 6B?) which functions as an intermediate frequency amplifierand as a detector and audio amplifier, (3) an audio frequency amplifierI5 (RCA Radiotron BL'I) interposed between the detector and audiofrequency stages of the device i4 and (4) an audio frequency amplifierI5 (RCA Radiotron 41) In the operation of the apparatus of Fig. 1,intermediate frequency signal impulses applied to the control grid 51 ofthe device I4 are amplified by this device and applied through anintermediate frequency transformer I8 to the detector electrodes Iii-20.The detector output circuit includes series resistor sections 21 and 22,and a cathode resistor 23 which are shunted by capacitors 24, 25 and 26respectively. From the upper terminal of resistor section 2|, an

automatic volume control bias potential is applied through a resistor 21to the control grids of the radio frequency stages I2 and. I3. A similarbias potential is applied from the upper terminal of the resistorsection 22 through a resistor 28 to an intermediate grid 29 of the audioamplifier I5. Signal input impulses are applied from the adjustableterminal 30 of the resistor section 22 through a capacitor 3! to theinput circuit of the audio frequency amplifier I5 from the outputcircuit of which the amplified signal impulses are applied through acapacitor 32 to the control grid ll of the device I4. These impulses areamplified by the device I4, are applied through a coupling assemblyincluding resistors 33 and 34 and a capacitor 35 to the input circuit ofthe audio frequency amplifier I6 by which they are further amplified andapplied through a coupling transformer 36 to the loudspeaker II.

It will be noted (1) that a capacitor 31 is provided to excludeintermediate frequency impulses from the input circuit of the device I6,(2) that a tone compensation circuit which includes the series resistor33 and capacitor 39 and the shunt resistor and capacitor 4! is connectedbetween the audio frequency output and input circuits of the device I4and (3) that the adjustable terminal 30 of the resistor section 22 andthe adjustable terminal 42 of a resistor 43 connected in the screen gridcircuit of the device I6 are arranged to be simultaneously adjusted by acommon control member 44.

In practicing the invention, the adjustments of the terminals 30 and 42are so related that the device l6 operates just below saturation and theknee of its characteristic curve, and thus by saturation the relativelyhigh amplitude noise impulses are limited to the signal level. It hasbeen found that the apparatus operates at substantially constant signallevel. Under these conditions, noise impulses of an amplitude exceedingthat of the signal are effectively limited by the device I6 and thiscondition of operation is readily attained at different operating levelsthrough the simultaneous adjustment of the terminals 30 and 42.

Considered apart from the elimination of noise, the above describedreflex circuit has the important advantage that it effects a greatimprovement in automatic volume control. Thus it has been found thatwith an input variation of 10,000 to 1 the inclusion of the automaticvolume controlled amplifier IS in the reflex circuit results in areduction of the output variation from 50 to 1 to 2 to 1, thus effectinga vast improvement in signal level stability.

The apparatus of Fig. 2 is similar in many respects to that of Fig. land will be readily under stood without detailed explanation. It differsin that (1) the audio frequency amplifier I5 is not included in thereflex circuit and the control grid of amplifier I6 is so biased as toobstruct the passage of relatively high amplitude noise impulses and (2)the tone compensation circuit is connected across the audio frequencyamplifier I6.

It will be observed that automatic volume control is applied to theamplifiers I2 and I3 from the upper terminal of the resistor section 2!through a battery 45 and the resistor 21; that audio frequency signalinput is supplied from the adjustable terminal to the input circuit ofthe device I4; and that the member 40 is utilized simultaneously tocontrol the adjustment of the volume or level control terminal 30 and ofthe terminal 46 of the input resistor 41 of the device l5. The operationof this apparatus is similar to that of Fig. l and will be readilyunderstood in view of the foregoing explanation.

- I claim as my invention:

1. A signal channel including serially connected amplifier tubes, meansfor controlling the gain of the first of said amplifier tubes, means foradjusting a bias potential of the second of said amplifier tubes tocontrol the saturation thereof, means for simultaneously operating saidcontrolling and adjusting means to maintain said bias potential at avalue such that relatively large amplitude noise impulses are limited bysaturation of the second tube to the level of the signal impulses, andadditional means for regulating the gain of the first of said amplifiertubes in accordance with variations in signal strength.

2. A signal channel including serially connected amplifier tubes, meansfor controlling the gain of the first of said amplifier tubes, meansincluding a screen grid electrode for adjusting a bias potential of thesecond of said amplifier tubes to control the saturation thereof, meansfor simultaneously operating said controlling and'adjusting means tomaintain said bias potential at a value such that relatively largeamplitude noise impulses are limited by saturation of the second tube tothe level of the signal impulses, and additional means for regulatingthe gain of said amplifier tubes in accordance with variations in signalstrength.

3. A signal channel including serially connected amplifier tubes, meansfor controlling the gain of the first of said amplifier tubes, means foradjusting a bias potential of the second of said amplifier tubes tocontrol the saturation thereof, means for simultaneously operating saidcontrolling and adjusting means to maintain said bias potential at avalue such that relatively large amplitude noise impulses are limited bysaturation of the second tube to the level of the signal impulses,additional means for regulating the gain of the first of said amplifiertubes in accordance with variations in signal strength, and tonecompensation means connected across one of said amplifier tubes.

4. The combination of an electron discharge device provided with aninput circuit including a cathode and control grid, with an outputincluding said cathode and an anode and with a detector circuitincluding said cathode and an auxiliary anode, means including a radiofrequency amplifier for energizing said input circuit, coupling meansinterposed between said output and detector circuits, means including anaudio frequency amplifier interposed between said detector and inputcircuits, and means associated with said'detector circuit forautomatically regulating the gain of said radio and audio frequencyamplifiers.

5. In a signal amplifying channel, the combination of an amplifiertubehaving a cathode, a control grid, an output anode, a screen gridbetween said control grid and output anode, and a diode rectifier anodeassociated with the cathode, means for applying modulated high frequencysignals to the control grid, means for coupling the output anode withsaid diode anode, an output resistor in circuit with said diode anodeand cathode, an electric discharge amplifier device having a cathode,,acontrol grid connected with said output resistor to receive rectifiedsignals therefrom, a second control grid connected with said outputresistor to receive a biasing potential therefrom responsive to signalstrength variations, and an output anode coupled to the control grid ofthe first named amplifier tube, to apply amplified and rectified signalsthereto, means for deriving said rectified and amplified signals fromthe output anode of said first named amplifier tube including an audiofrequency amplifier tube having a screen grid, and means forsimultaneously adjusting the potential of said screen grid and thesignal potential applied to the control grid of said second namedelectric discharge device.

6. In a signal amplifying system, the combination of an electricdischarge amplifier device including a diode rectifier, means forapplying signals to said amplifier device, means for applying amplifiedsignals from said device to said rectifier, an audio frequency amplifiercoupled'to said first named amplifier device to receive rectified andamplified audio frequency signals therefrom, means for controlling thegain of said amplifier devices in accordance with the variations in thevalue of the signal voltage derived from said rectifier, and means formanually adjusting the gain controlling potential applied to said firstnamed amplifier device and the signal potential applied to said secondnamed amplifier device in predetermined relation to each other.

ROGERS M. SMITH.

